SR 520 Bridge Replacement and HOV Program - Light rail in the corridor

High-capacity transit on the new SR 520 floating bridge

In 2008 WSDOT, King County Metro and Sound Transit, in cooperation with the University of Washington, developed a transit strategy for SR 520, outlined in the SR 520 High Capacity Transit Plan.

The strategy includes a network of bus rapid-transit lines between Seattle and the Eastside on SR 520’s new HOV lanes. When fully reconstructed, the highway will have bus/carpool lanes from I-5 to Redmond. Also part of the transit plan are rail-to-bus transfers now provided at the new Montlake Multimodal Center that opened in 2015 next to Sound Transit’s University Link light-rail station. The U-Link station, near Husky Stadium and the UW Medical Center, opened in early 2016.

In addition to bus rapid transit on SR 520, voters in the Puget Sound region approved Sound Transit 2 in November 2008. Most notably, the measure approved funding for extending light rail across Lake Washington on I-90. In addition, Sound Transit 2 funds more than 100,000 additional transit-service hours to further develop bus rapid transit in the SR 520 corridor.

In December 2014, Sound Transit updated its long-range plan for regional mass transit. The new plan identifies SR 520 as a potential corridor for further expansion of high-capacity transit to meet future growth in the region.

Light rail on the new SR 520 floating bridge

WSDOT engineers have designed the new SR 520 floating bridge so that additional supplemental pontoons could be added in the future to support the weight of light rail. As shown in the graphics below, light rail could be accommodated either by converting the bridge's transit/HOV lanes to light rail, or by adding more width to accommodate light rail in each direction along with the four general-purpose and two transit/HOV lanes.

The cost to add light rail to the new floating bridge

In terms of time, the state Attorney General’s Office and the Federal Highway Administration estimate that it would take two years to conduct the required environmental analysis to add light rail to the new SR 520 floating bridge. This would be essential since the current analysis assumes four general-purpose lanes and two transit/HOV lanes on the bridge, and does not address light rail.

Secondly, it would cost between $150 million and $200 million to construct the 30 additional pontoons and to install them on the floating bridge, alongside the 77 pontoons required for the six-lane bridge. There would be other costs associated with the bridge deck expansion and other infrastructure, including rail lines.

SR 520 bridge layout diagrams

The planned SR 520 bridge

With two general-purpose lanes and one transit/HOV lane in each direction.

Potential SR 520 bridge configurations that accommodate light rail

Scenario 1: With light rail displacing the transit/HOV lanes.

Scenario 2: With light rail in addition to the transit/HOV lanes.

Note: Each scenario would require WSDOT to construct an additional 30 pontoons to support the weight of light rail. Adding light rail to the SR 520 floating bridge would also require analysis of transit connections and routes, additional funding, regional decision-making, and a separate environmental review process.